The present invention relates generally to joint detection of data signals and, in particular, to joint detection of data symbols simultaneously transmitted over a shared communications channel and distinguished by user-specific signature or spreading sequences.
Since the early 1990""s, code division, multiple access (CDMA) cellular communications systems have been designed based on direct-sequence spread-spectrum (DS-SS) principles. In such systems, multiple users simultaneously occupy the same radio frequency channel, separated only by user-specific spreading or signature sequences. Probably the best known contemporary example of this approach to cellular system design is that defined by Personal Station-Base Station Compatibility Requirements for 1.8 to 2.0 GHz CDMA Personal Communication Systemsxe2x80x9d (American National Standards Institute (ANSI) J-STD-008), or, broadly equivalently, the Telecommunications Industry Association Interim Standard 95 (TIA IS-95).
Recently, proposals for so-called xe2x80x9c3-rd Generationxe2x80x9d cellular communication systems have been made to the European Telecommunications Standards Institute (ETSI) for adoption within the Universal Mobile Telecommunications System (UMTS) standardization process. Among the proposals are systems based on Direct-Sequence, Spread-Spectrum (DS-SS) technology. One such candidate system, which is particularly relevant to the present invention, is usually referred to in the UMTS community as the TD-CDMA system (for xe2x80x9cTime-Division Code-Division Multiple Accessxe2x80x9d). That system makes use of a combination of time- and code-division techniques as a means of improving overall system capacity while purportedly retaining compatibility with 2nd Generation systems, notably the Group Special Mobile (GSM) cellular communications system.
As can be seen from FIG. 1, the TD-CDMA air interface incorporates both TDMA and CDMA elements. The TDMA component is provided by dividing each radio frequency channel into frames of 4.615 ms duration, with each frame further divided into time slots of approximately 577 us in length. The CDMA element is permitted by the allocation to each user of a unique 16-ary Walsh-Hadamard orthogonal spreading code which is used to spread the Quadrature Phase Shift Keyed (QPSK) or Quadrature Amplitude Modulation (16-QAM) data symbol sequences comprising the useful part of each user""s transmission.
The mobile station (MS) or base station (BS) transmission within any timeslot is referred to a xe2x80x98burstxe2x80x99. As presently envisaged, the TD-CDMA air interface supports two distinct burst types, the general structure of which is shown in FIG. 1. The so-called type-1 burst transmits a data sub-burst of 28 data symbols, followed by a midamble of length 296 chips (used for channel estimation purposes), a second sub-burst of 28 data symbols, and finally a guard period of 58 chips; the type-2 burst transmits 34 symbols in each sub-burst, with midamble and guard period durations of 107 and 55 chips respectively. In both cases the length of each burst is 1250 chips. The same burst structures are used for both forward and reverse links, although since the forward link represents a point-multipoint transmission, the content of the midamble segment is slightly different in that case. This is of no consequence, however, for the present purposes. Accordingly, the description of the drawings below will focus on reverse link operation, pointing out differences between the forward and reverse only when these are significant for the description of the invention.
FIG. 2 shows a communication system employing a TD-CDMA air interface. A single timeslot is shown by the figure, within which K mobile stations 201-209 are simultaneously active. In the example shown, the mobile station population 201-209 transmits simultaneously on a specific timeslot, distinguished by length-16 spreading codes c(1) through c(k) respectively. It is important to note thatxe2x80x94unlike IS-95 systems in which the sequence used to spread each symbol is a sub-sequence of a much longer sequencexe2x80x94the same code c(k) is used continuously to spread each data symbol from the same user. In other words, the received signal at BS 200 comprises a plurality of time overlapping coded signals from individual mobile stations, each transmitted within the same timeslot and distinguishable by a specific signature sequence.
The use of conventional DS-SS receivers such as the RAKE receiver is not, however, envisaged for use in the TD-CDMA system. Rather, receiver designs capable of simultaneously or jointly recovering the data symbols transmitted by the population of mobile stations operating within the same timeslott frequency within a given cell or sector are intended. Examples of such receivers are described in articles Lupas R., Verdu S., xe2x80x9cLinear Multiuser Detectors for Synchronous Code-Division Multiple-Access Channelsxe2x80x9d, IEEE Trans. Inf. Theory, vol. 35, no. 1, January 1989, Klein A., Baler P. W., xe2x80x9cLinear Unbiased Data Estimation in Mobile Radio Systems Applying CDMAxe2x80x9d, IEEE J. Sel. Areas Comm., vol. 11, no. 7, September 1993, Blanz J., Klein A., Nashan M., Steil A., xe2x80x9cPerformance of a Cellular Hybrid C/TDMA Mobile Radio System Applying Joint Detection and Coherent Receiver Antenna Diversityxe2x80x9d, IEEE J, Sel. Areas Comm., vol. 12. No. 4, May 1994, and Jung P., xe2x80x9cJoint Detection with Coherent Receiver Antenna Diversity in CDMA Mobile Radio Systemsxe2x80x9d, IEEE Trans. Veh. Tech., vol. 44, no. 1, February 1995.
The receivers described in the above-mentioned references perform joint detection using block linear joint sequence detectors. The computational complexity of the methods used to solve the system of equations arising from the above methods can, however, be very large. This results in receivers that are costly and consume large amounts of power. Therefore, a need exists for a method and apparatus for joint detection of data signals that requires less computational complexity, resulting in less costly, lower power receivers.
Put more generally, a need exists for a method and apparatus to reduce the computational complexity of linear joint detectors where the signature sequences of the users are constant over a block (or burst) of symbols, and where some external means are providedxe2x80x94in the form, for example, of a midamble or pilot sequencexe2x80x94for performing channel estimation.